1. Field of the Invention
The present invention relates to a free wheel, that is, an element which couples two mechanical parts so that the two parts can have a relative rotation in only one direction.
The present invention more particularly concerns a disengageable free wheel allowing, depending on the active or inactive state of a control signal, two parts to rotate one with respect to the other, either in two directions, or only in the one direction determined by the free wheel, hereinafter the "free" direction.
2. Discussion of the Related Art
A particularly interesting application of a disengageable free wheel is described in French patent application 2,691,093 (filed on May 12, 1992) relating to a surgical robot for guiding movements. In such a robot, each articulation must be blockable, capable of freely rotating in two directions, or capable of retating only in a selected one of the two directions. These four possible states of the articulation are determined by logic control signals. To obtain these four articulation states, two coaxial disengageable free wheels, with opposite free directions, are used.
FIG. 1 represents a conventional free wheel 8. The free wheel 8 includes an internal shaft 10 surrounded by an external ring 12. The inner wall of ring 12 includes a plurality of ramps 13 oriented in a same direction. In FIG. 1, each ramp begins at a position near shaft 10 and departs therefrom clockwise. A roller 14 is associated with each ramp. The diameter of roller 14 is slightly smaller than the distance separating the periphery of shaft 10 from the most distant portion of the ramp. Moreover, as represented in FIG. 1, the top 13-1 of each ramp is rounded to match the associated roller 14.
The operation of this free wheel is as follows. Assume that shaft 10 is fixed. When ring 12 is rotated clockwise, the ramps 13 start to roll on rollers 14 like in a ball bearing. Almost immediately, depending on the clearance about rollers 14, the diameter of roller 14 becomes smaller than the gap separating the periphery of shaft 10 from ramps 13. Rollers 14 then block and prevent ring 12 from continuing to rotate clockwise. The free wheel blocks rapidly so that the clockwise rotation (in the non-free direction) of ring 12 is negligible.
In contrast, when ring 12 is rotated counterclockwise (as indicated by an arrow S in FIG. 1), rollers 14 are urged back to the top of ramps 13 where they can freely rotate.
Of course, other types of free wheels are available, for example ratchet free wheels. However, the drawback of such ratchet free wheels is to allow a slight rotation in the non-free direction before they block.
A conventional solution to manufacture a disengageable free wheel consists in providing a clutch for coupling or uncoupling, for example, shaft 10 of the free wheel with a part whose rotation should be controlled with respect to ring 12. If the shaft is uncoupled, the part can rotate in any direction with respect to ring 12. If the shaft is coupled, the part can rotate in the free direction only.
Such a clutch device always has some clearance between the coupled and uncoupled positions, which causes high response delay time and noisy operation. A high response delay time when coupling (blocking) is hazardous for a surgical robot. When an articulation of the robot reaches a limit angular position, it must be stopped (by coupling it with a corresponding free wheel). If the coupling response delay time is long, the limit may be exceeded before the articulation is stopped.
Moreover, the blocking of the articulation must be immediate to ensure satisfactory accuracy. It is difficult to provide slowing down the articulation by friction effect; indeed, this would cause an inaccurate final position depending on the energy provided to the articulation.